Hi Mel,
Not quite the same situation as yourself as I'd travelled solo before I discovered JY but will say as a company they do look after you on the holidays and are very helpful if you have any querries.
As for America they have some great choices, I've done two with them so far "Americas Golden West" and "Parks & canyons", with "Route 66" coming up in October.
Loved both the trips to the States so far thats why i keep coming back, Golden West was my first JY holiday and still a favorite.
Most holidays have a good mix of experienced travellers and nervous newbies like yourself, just pick what you fancy and go for it, very much doubt you'll regret it.
Mick